Conventional treadmills employ a motor to rearwardly drive an endless belt. Generally, a user of a conventional treadmill is able to vary the speed and incline of the treadmill to obtain a desired level of workout. More sophisticated treadmills, such as described in U.S. Pat. No. 5,462,504 and assigned to the assignee of the present application, automatically adjust the speed and incline of the treadmill to control the heart rate of the user.
In general, treadmills function to exercise the user's cardiovascular system and the skeletal muscles of the lower body, but do not exercise the upper body to any significant extent. Accordingly, a number of treadmills have upper body exercise devices associated therewith, such as arm members which are moveable against the resistance of a spring.
While conventional motor-driven treadmills provide a desirable exercise apparatus in appropriate settings, in other settings the motor makes such an apparatus undesirable. For example, motors need maintenance, can fail, need to be plugged into a power source and add to the weight of the treadmill, while the expense of the motor is a major factor in treadmill purchases.
As a result, simple treadmills are known which do not use motors, but are instead designed to be inclined such that the belt rotates rearwardly as a result of the weight and forward stride of the user overcoming belt friction. However, once the incline is set, these types of treadmills feel unnatural to a user because changes to the belt speed depend upon the amount of additional rearward force a user is able to apply. For example, without interrupting an exercise session to adjust the incline, a user wishing to increase the speed of a gravity-driven belt must push down and/or forwardly on hand rails or arm members in order to change the amount of rearward force applied to the belt. Such a workout is not at all like a person's natural stride when increasing or decreasing speed.